86 A VOYAGE TO Book VII, 



in the earth is comparatively but frnall. Whence it 

 is inferred that an aperture being once made, how- 

 ever the fubftanccs in the bowels of the mountain 

 may take fire, the convulfion of the earth is feldom 

 or never felt a fecond time. The reafon of which is, 

 that the fudden reiteration of this accident greatly 

 augments the volume of the air by rarefaction, and as 

 it finds an eafy palTage without labouring in the 

 bowels of the earth for a vent, no other conculTion 

 is produced than what muft follow from the: eruption 

 of a great quantity of air through an aperture too 

 narrow for its volume. 



The formation of volcanoes is now well known; 

 and that they owe their origin to fulphureous, ni- 

 trous, and other combuftible fubftances in the bow-, 

 ds of the earth ; for thefe being intermixed, and, as it 

 were, turned into a kind of pafte by the fubterraneous 

 waters, ferment to a certain degree, when they take 

 fire, and by dilating the contiguous wind or air, 

 and alfo that within their pores, fo that its volume 

 is prodigioufly increafed beyond what it was before 

 the inflammation, it produces the fame cffed as 

 gunpowder, when fired in the narrow fpace of a 

 mine; but with this difference, that powder on be- 

 ing fired immediately difappears, whereas the vol- 

 cano being once ignited continues fo till all the 

 oleaginous and fulphureous particles contained in the 

 mountain are confumed. 



Volcanoes are of two kinds, contra6led and di- 

 lated. The former are found where a great quan- 

 tity of inflammable matter is confined in fmall 

 fpace; the latter where thefe combuftibles are fcat- 

 tered at a confiderable diflance from one another. 

 The firft are chiefly contained in the bowels of 

 mountains, which may be confidered as the natural 

 depoiitaries of thefe fubflances. The fecond may be 

 confidered as ramifications, which, tho' proceeding 

 from the former, are, however, independent, exr 



tending 



